This invention relates to a method and apparatus for removing and installing surface mounted devices (SMD's) with respect to a substrate such as a printed circuit board (PCB).
One problem, especially in the field of rework and repair of PCB's, is that of aligning the leads or terminals of an SMD with respect to the lands or pads on a board where, for example, the SMD is to be installed on the board in place of a defective SMD which has previously been removed therefrom. (Of course, this problem does not arise when an SMD is being removed since it is already attached to the board.) That is, the tendency with more recent SMD's is to increase the number of leads per side where, in the future, there may be as many as 100 leads per side and where the spacing or pitch between leads may be 1 to 2 mils. At present, a spacing of 40 to 50 mils may be considered coarse spacing, a spacing of 20 to 30 mils may be considered medium, while a spacing of 10 to 15 mils may be considered fine.
Techniques are known for effecting alignment of SMD leads with respect to PCB lands, which techniques are appropriate for alignment of SMD's with coarser spacing of terminals. However, with finer spacing, more precise techniques are required and these are provided in the present invention.
Techniques are known in the mass production of PCB's and the like whereby SMD leads may be accurately aligned with respect to lands on the PCB. However, the techniques involve very precise, computer-controlled elements for effecting this alignment, which, in turn, involves substantial expense. This expense is, of course, justifiable in the mass production field; however, in the field of rework and repair, where, in many instances, an individual defective SMD is being replaced, such expense usually cannot be justified. Accordingly, it is a primary object of the invention to provide an accurate alignment capability of the above type appropriate to the field of rework and repair.
Another consideration relating to accurate alignment of SMD leads with respect to PCB lands is that of the particular type of SMD being installed. That is, SMD's may be divided into two groups depending on the type of leads employed. In the first group, the leads may be characterized as relatively rigid although there may be some flexibility. The carriers for such leads typically include (a) so-called leadless carriers where the leads are disposed in castellations formed on the sides of the carriers, the leads then extending beneath the carrier where they make contact with the lands in the PCB, (b) J-type leaded carriers where the leads have a J-type configuration and extend from the carrier side to beneath the carrier and (c) some gull-wing leaded carriers such as the Flat-Pac type where the leads extend outwardly from the carrier and terminate at some point removed from the carrier side not beneath the carrier. The foregoing lead configurations employed with integrated circuit carriers are well-known and are discussed in more detail in above-mentioned U.S. Pat. No. 4,659,004 and Application No. 092,469.
The second group of integrated circuits comprises gull-wing leaded carriers (hereafter sometimes refered to as Flat-Pac's) where the leads are substantially less rigid than those of J-type leaded carriers and leadless carriers. As indicated above, some Flat-Pac's may be included within the first group if the leads thereof are sufficiently rigid. This, to a certain degree, depends on the number of leads per side, the dimension of the leads and the concomitant spacing between the leads. That is, as the number of leads increases, the leads grow smaller in size and thus tend to become substantially less rigid. This decrease in the lead size and the spacing therebetween renders it substantially more difficult to align the leads and lands. Moreover, as discussed above, the tendency is toward even a greater number of leads per side and hence, the need for a device which can accurately align such SMD's can be appreciated.
In accordance with a primary aspect of the present invention, a viewing station is provided which is separate from the reflow station where the SMD is attached to the board. That is at the viewing station, substantially unrestricted vertical or Z-axis viewing capability is provided which may be implemented either with the aided or unaided eye (that is, with or without a microscope or a video monitor, for example). Once the SMD is aligned with the board (and semi-permanently attached thereto), it may be moved to the reflow station where the SMD is soldered to the board. Solder reflow is preferably effected by a heated air devioe such as those disclosed in above-mentioned U.S. Pat. Nos. 4,605,152 or 4,659,004 although other reflow devices such as those which bring a heated element into direct contact with the leads may also be used, an example of such a device being disclosed in U.S. Pat. No. 3,382,504, which is incorporated herein by reference. Other heated air devices are disclosed in U.S. Pat. Nos. 3,524,247; 4,295,596; 4,426,571; 4,444,559; 4,561,584; and 4,561,135. Alternatively, rather than moving the aligned SMD to the reflow station, the reflow station may be moved to the aligned SMD at the viewing station.
As discussed above, the first group of integrated circuits may be characterized by the relative rigidity of the leads thereof. This rigidity is helpful in attaching SMD's such as J-type leaded carriers to the board since, once alignment has occurred (with the SMD being held slightly spaced from a finely movable PCB), the J-type leaded carrier leads may then be depressed onto the respective lands with substantially no danger of the leads slipping off the lands. In this regard, it should be appreciated it is necessary to depress the leads on the lands since bottom of the leads of a J-type leaded carrier are typically not co-planar. Once depressed onto the PCB leads, the reflow station may be lowered into position to solder the J-type leaded carrier to the PCB.
Hence, in accordance with a further aspect of the invention, if attachment of SMD's belonging only to the above-discussed first group is contemplated, both the alignment and reflow functions may be effected at the reflow station where the alignment is generally effected with more accuracy than with prior art devices.
The second group of SMD's (Flat-Pac's with substantially less rigid leads) do not lend themselves to forceful depression on the lands because they tend to slip off the lands due to their lack of rigidity. Hence, in accordance with a further aspect of the invention, means are provided whereby the leads of such Flat-Pac's may be lightly positioned on the lands at the above-mentioned separate viewing station subsequent to alignment of the leads with the lands. Prior to the positioning of the leads on the lands, the lands may be cleaned and repaired (if necessary, due to the previous removal of the defective component), pretinned, and have a tacky flux applied thereto. Thus, when the SMD is positioned on the lands, it is semi-permanently held in place until it can be soldered to the board at the reflow station Hence, this capability of lightly positioning the second group of SMD's on the board at the separate viewing station is a further advantageous feature of this separate station although this capability could, with additional complexity, also be provided at the reflow station.
Flat-Pac's (with less rigid leads) are becoming increasingly popular because it is easier to align these carriers with the board lands since the leads do not terminate beneath the carriers as do the leads of leadless carriers and J-type leaded carriers. Hence, the capability by the present invention of aligning and then installing SMD's of the second group by providing a separate viewing station to effect alignment is also advantageous in that it addresses this trend toward increasing use of Flat-Pac's.
Heretofore, the alignment function described has been that of aligning SMD leads with respect to PCB lands. Not only should this alignment function being accurately effected but also alignment of the SMD with respect to the nozzle of the heater unit. That is, during reflow, as is known from aforementioned U.S. Pat. Nos. 4,605,152 and 4,659,004, the lower edge of the nozzle is slightly spaced from the PCB and the SMD is preferably coaxially disposed within the nozzle such that each side of the SMD is equally spaced from its associated wall of the nozzle.
When alignment of the SMD with respect to the PCB (as opposed to with respect to the nozzle) occurs at the reflow station, as discussed above, alignment of the SMD with respect to the nozzle is preferably effected as described in aforementioned Application No. 78,170 although other means may be employed as described in aforementioned U.S. Pat. No. 4,715,640.
In those embodiments where the alignment of the SMD occurs at a viewing station separate from the reflow station, means are provided, in accordance with the invention, to effect alignment of the SMD with respect to the nozzle after alignment of the SMD with respect to the PCB has been effected. In general, in accordance with the invention, a positionable worktable, which supports the PCB, is translated a predetermined horizontal distance between the reflow station and the viewing station where the alignment of the SMD with respect to the PCB is effected. Positionable tables of the above type are known and are described in aforementioned U.S. Pat. No. 4,682,766 and Application No. 914,921. However, the positionable table of the present invention include certain improvements with respect to the earlier tables Moreover, the capability of translating such tables a predetermined distance from the reflow station is not known and facilitates the performance of a number of functions associated with the replacement of a new SMD on a PCB. Thus, in those embodiments where the positionable table is translatable to the viewing station, the board may be so translated to the viewing station, subsequent to removal of a defective SMD, where any damage caused to the lands or board by removal of the defective SMD may be repaired The PCB may then be prepared for installation of the new component by cleaning it and applying a tacky flux which will facilitate semi-permanent attachment of the SMD once it has been aligned with the PCB lands. Moreover, the leads of the SMD and/or the lands of the PCB may be tinned at this time Next the SMD is aligned with the board, the SMD being slightly spaced from the board at this time. Next, the SMD is lightly positioned on the aligned PCB lands to ensure the leads to not slip off the lands. The so positioned SMD is then inspected to ensure alignment has been maintained. The viewing means whether they comprise the aided or unaided eye may and normally would be the same viewing means employed to effect alignment of the SMD with the board.
Next, the aligned SMD must be returned to the reflow station or the reflow station moved to the aligned SMD such that the SMD is now aligned with the reflow station nozzle. In one embodiment of the new SMD needs only to be translated back through the above-mentioned predetermined distance to the reflow station where it will be now precisely aligned with the nozzle. This is effected by accurately positioning the SMD at the viewing station prior to the alignment procedure, the new SMD being held in place while the position of the board beneath it is adjusted until alignment occurs. Thus, as stated above, the board need only be translated back through the predetermined distance to the reflow station to effect alignment of the new SMD with the nozzle.
If the new SMD is not accurately positioned at the viewing station prior to the alignment thereof with the board, means are provided in accordance with another aspect of the invention where the aligned, semi-permanently attached SMD may be re-positioned to a predetermined position such that translation of the board through the predetermined distance will result in alignment of the new SMD with the nozzle. Accordingly, a mechanical or optical reticle may be employed to define the above predetermined position on the board. By use of one of the above reticles, the position of the board with the newly aligned SMD thereon may be adjusted until the SMD is aligned with the predetermined position defined by the reticle. Once the SMD is positioned in the predetermined position, the board may again be translated back through the predetermined distance where the SMD will now be aligned with reflow station nozzle. The nozzle may now be lowered on the SMD whereby reflow may be effected to solder the SMD to the board. After the SMD has been soldered to the board, the board may again be translated to the viewing station for a final inspection to insure the SMD has been properly soldered to the board.
In the embodiments of the invention where the processed SMD's are such that it is not necessary to provide a separate viewing station for aligning the SMD with respect to the board, it is still desirable to provide the above capability of translating the board a predetermined horizontal distance from the reflow station to thereby implement many of the functions described above. In this regard, it should be noted that the direction of the predetermined horizontal distance may either be to the side of or in front of the reflow station. In the embodiment described hereafter, the direction of translation of the board is to the side of the reflow station. However, in those embodiments where a separate viewing station is not needed for alignment, the direction of translation may preferably be in front of the board.
Moreover, in the last-mentioned embodiments, alignment of the SMD with respect to the nozzle may be effected by known means as discussed above. Typically, these means include locating means disposed within the nozzle such that the SMD is manually inserted into the nozzle. Once the SMD has been inserted in the nozzle, alignment of the SMD with respect to the PCB may now be effected, in accordance with a further novel aspect of the invention, by attaching the inserted SMD to a vacuum tube which coaxially extends through the heater assembly and which is independently movable with the heater assembly such that the SMD may be lowered to and held at a substantial distance such as 11/2 inches beneath the nozzle where alignment thereof may be effected by viewing means such as the microscope of Patent Application No. 921,220 mentioned above. The above-mentioned U.S. Pat. No. 4,605,152 discloses a heater assembly which is capable of vertical movement and a vacuum tube which coaxially extends therethrough. Moreover, there is mentioned therein that the vacuum tube may be used for positioning an electronic component on a substrate. However, in actuality an alignment plate is used for the alignment function, which has not proven satisfactory in many instances. Furthermore, there is no mention in the last-mentioned patent that the vacuum tube with an SMD attached thereto may be lowered to a position substantially beneath the nozzle to facilitate alignment of the SMD with the PCB and, in particular, there is no disclosure of means in the latter patent of holding the SMD in such a lowered position.
Thus, in those embodiments where a single station is employed for both aligning the SMD with respect to the PCB and for reflow, it is still desirable to provide a separate viewing station for effecting the following functions. That is, once the defective component is removed, the board may be translated in a single movement to a separate station where any damage which occurred to the lands or the board may be repaired. Cleaning and pretinning, as required, may also be effected. The board may then be translated back to the reflow station where alignment of the SMD with respect to the PCB and then attachment thereto, in a manner to be described below, may be effected. Once the SMD has been soldered to the board, the board may again be translated to the separate station for a final inspection.
In accordance with a further aspect of the invention, alignment of the SMD with respect to the board is accomplished at the reflow station (as opposed to a separate viewing station) in the following manner. First the new SMD is inserted in the nozzle to effect alignment of the SMD with respect to the nozzle due to locator means in the nozzle as described above. Next, the above-mentioned vacuum tube is attached to the SMD and then the tube is lowered to and held at a position substantially below the heater assembly nozzle and slightly above the board whereby the position of the positioning or work table supporting the PCB is adjusted until accurate alignment of the SMD leads with respect to the PCB lands is effected typically utilizing the above-mentioned optical aids. Once alignment is effected, the vacuum tube is further lowered to press the leads against the lands and thus ensure good mechanical and eventual electrical connection between all leads and their associated lands. Next, the heater assembly including its nozzle is lowered to the PCB so that the nozzle is in reflow relation to the SMD. Reflow is next effected by delivering heated air to the nozzle to thus solder the SMD to the PCB. The vacuum tube may then be removed from the SMD and the heater assembly raised. The attached SMD may then be translated to the separate station for final inspection as discussed above.
In accordance with a further aspect of the invention, alignment of the SMD with respect to the board is accomplished at the separate viewing station (as opposed to the reflo station) in the following manner. Subsequent to the removal of a defective SMD at the reflow station, the PCB is translated a predetermined distance to the viewing station preparatory to the alignment of the new SMD with the PCB. The new SMD is inserted into a cup-shaped member, the internal configuration of which corresponds to that of the new SMD. The cup-shaped member is removably attachable to a tubular vacuum pick whereby vacuum is applied to the interior of the cup-shaped member to secure the SMD therein. The leads of the SMD extend beyond the periphery of the cup-shaped member to thus permit alignment of the leads with the lands on the PCB where substantially unobstructed vertical viewing of all four sides of the SMD is available. Different cup-shaped members may be connected to the vacuum tube depending upon the particular SMD being processed. Preferably the vacuum tube is rotatable from a first position which is in alignment with the nozzle of the heater at the reflow station to a retracted second position where the new SMD can be inserted either manually or automatically into the cup-shaped member. In automatic operation, the cup-shaped member may remove the SMD's from a chute or the like.
The first position of the vacuum tube corresponds to an aligned position with respect to the nozzle of the heater assembly, as stated above. In particular, the substrate work holder is translatable from the reflow station to a position in rather precise alignment with the first position of the vacuum tube or pick. With the new SMD inserted in the cup-shaped member, the alignment of the new SMD with the lands on the PCB may be effected by adjusting the position of the work table until alignment occurs. This is facilitated, especially when the number of leads is very large, as discussed above, by permitting substantially unobstructed vertical viewing of all four sides of the new SMD. Alignment is effected while the spacing between the new SMD and the lands on the PCB is very slight, preferably substantially less than 1/16 of an inch.
As discussed above, prior to the alignment procedure, the lands on the PCB may be cleaned and pretinned, if necessary, while the vacuum tube is in its retracted position Also, a tacky flux is applied to the lands at this time, such tacky fluxes being known and used in the above-described mass production processes.
After alignment has occurred, the new SMD is gently placed on the lands of the PCB by removal of the vacuum applied thereto or gently lowering the SMD onto the PCB lands.
Alignment of the SMD with the PCB having now been accomplished, alignment of the SMD with the nozzle of the heater assembly must now be effected. Assuming the SMD had been properly centered within the cup-shaped member, it can be assumed that the SMD will be properly aligned with the reflow nozzle when the substrate is moved back to the reflow station since means, in accordance with the invention, are provided to ensure precise movement of the worktable between the reflow station and the viewing station. Alignment of the SMD within the cup-shaped member can be sufficiently ensured by providing locating means within the cup-shaped member similar to that discussed hereinbefore within the heater assembly nozzle. If such locating means are not employed, additional means are provided, in accordance with a further aspect of the invention, to move the new SMD to a further position from which the work table can now be translated back to the reflow station nozzle with assurance that the new SMD will be aligned with the nozzle at the reflow station. Accordingly, a mechanical reticle is provided on a vertically extending portion of the vacuum tube, the reticle comprising a transparent membrane with a plurality of grid lines which correspond to the configurations of the various SMD's that may be processed with the invention. The reticle is precisely positioned on the vacuum tube so that when the new SMD is aligned with the reticle grid line corresponding to the shape thereof, the SMD will be aligned with the heater assembly nozzle when it is returned to he reflow station. Alignment of the SMD with the reticle holes is effected simply by the operator viewing the SMD through the reticle holes and positioning the work table until alignment is effected. The SMD may now be returned to the reflow station. The nozzle of the heater assembly is then lowered around the SMD to solder the SMD to the PCB in the manner described above. After the SMD has been soldered to the PCB, the heater assembly is raised and the SMD is again returned to the viewing station where all four sides thereof are inspected to ensure proper connection of the SMD to the board. It is particularly important in this final inspection that unobstructed, vertical viewing of all four sides of the component be available and, as can be appreciated from the foregoing, this is available with the present invention whether the alignment occurs either at the reflow station or at the separate viewing station.
A particular problem relating to hot air SMD removal and replacement devices of the type described in aforementioned U.S. Pat. Nos. 4,605,152 and 4,659,004 arises because PCB's with different thermal characteristics are processed by such devices. That is, due to the nature of the construction of the board (ceramic board, for example) and the presence of heat sinks, a large amount of heat may be required to raise the solder to its reflow (melting) temperature to thus permit either removal or installation of an SMD. Other boards (such as simple double-side PCB's with glass substrates) tend to drain relatively little heat and thus less heat is needed to raise the solder to its reflow temperature. Thus, in the first instance, the temperature and/or flow rate of the heated air should be greater than that in the second instance.
In certain prior art devices such as those disclosed in the foregoing patents, the temperature of the air is fixed at a level which will provide sufficient heat to effect reflow even if a board with large heat sinking capability is being processed. However, this presents a problem if a board with low heat sinking capability is being used and removal of a defective SMD is being effected. That is, normally, once the operator observes solder melt, he effects removal of the SMD by manual actuation of a vacuum pick attached to the component as described in aforementioned U.S. Pat. Nos. 4,605,152 and 4,659,004. However, if the operator delays several seconds after solder melt to remove the component, there is a danger the board will overheat thereby damaging the board and possibly adjacent good components. If the board has high heat sinking capabilities this danger is usually not significant. However, if it has low heat sinking capabilities, this danger becomes quite probable.
Accordingly, in accordance with another aspect of the invention, means are provided whereby the temperature of the hot air delivered to the SMD leads may be initially set to a value depending on the type board being processed. Thus, with a board having a lower heat sinking capability, the temperature of the hot air may be set to a lower temperature than that set for a board having a higher heat sinking capability.
As disclosed in the aforementioned U.S. patent application No. 92,469, different size and shape nozzles may be employed depending on the type of SMD being processed where the size of nozzle employed with a gull wing leaded carrier such as a Flat-Pac may be larger than that used for a leadless or J-shaped leaded carrier. Due to the different kinds of nozzles, different flow rates of the heated air by the leads to be soldered or desoldered result. Due to these different flow rates, different amounts of heat are delivered to the components leads--the faster the flow rate, the more the heat. Since the heat should be so delivered to the component leads that the solder is raised to its reflow temperature for rapid removal of the SMD while avoiding damage to the SMD or the PCB, it is desirable that the flow rate be adjustable depending upon the type nozzle employed.
In accordance with a further aspect of the invention, the flow rate of the heated air can be adjusted according to the nozzle type and board types. Moreover, due to the aforementioned capability of setting and controlling the temperature of the heated air, the temperature of the air is maintained substantially at the set temperature regardless of the foregoing adjustments of the air flow rate.
In a first, preferred embodiment of the invention, two temperature sensors are utilized in the hot air delivery device, this device generally including an upper chamber containing a heater by which flows air to be heated and a nozzle removably connected to the upper chamber for delivering the heated air to the SMD being processed, such devices being described in aforementioned U.S. Pat. Nos. 4,605,152 and 4,659,004. As will be described below, the hot air delivery device of the present invention includes a number of improvements with respect to those disclosed in the above patents. The first of the above sensors is located at the heater itself to permit idle temperature control of the heater while the second sensor is disposed in the heated air stream relatively close to the nozzle to permit more accurate control of the temperature of the air delivered to the component leads.
Aforementioned U.S. Pat. No. 4,605,152 discloses a sensor for sensing the temperature of the air passing through the heating device; however, the sensor is simply used to maintain the heater at a constant temperature. Thus, this use of the sensor is quite different from that of the second sensor the present invention whereby the temperature of the heated air is more accurately controlled.
Moreover, in accordance with a further aspect of the invention, the second sensor adjustably controls the heater temperature only during a reflow cycle during which solder melt is effected. At all other times (that is, during idle times), the heater idle temperature is controlled by the first sensor to maintain the idle temperature at a value whereby when unheated air is initially flowed by the heater during the reflow cycle, the air temperature will be quickly brought to the temperature controlled by the second sensor.
In accordance with a further aspect of the invention, spring means are provided for quickly attaching the nozzle to the heter assembly and precisely locking in place with respect to the heater assembly Moreover, the same spring means, in accordance with the invention, may be used to facilitate rotation of the nozzle for shearing a desoldered SMD from the PCB. The nozzle is then precisely returned to its aligned position with respect to the heater assembly
These and other objects of the invention will be apparent from the following detailed description and the drawing.